Help, My daughter decided to set up a 100 litre tank for Figure 8s. She's put 3 in the tank and all 3 have died with the last one being last night. I'm trying to understand what the sensitivity is causing these to die as they all have died overnight. Could it be a PH drop or is it just coincidental. She's considering packing it in but I'm trying to encourage her to carry on if we can just understand the underlying issues. Any thoughts?